System and method for detecting adverse atmospheric conditions ahead of an aircraft

ABSTRACT

The system has multiple, infrared cameras  8  adjusted to spatially detect infrared radiance in different bands of infrared light, and connected to an image processing computer that processes and combines the images, and generates video display signals for producing a video display which indicates the position of the adverse atmospheric conditions relative to the aircraft. Each of the cameras is provided with a respective filter adjusted to filter infrared light with a bandwidth corresponding to infrared bandwidth characteristics of an adverse atmospheric condition from a set of adverse atmospheric conditions. The image processing computer is adapted to identify adverse atmospheric conditions, based on threshold conditions and using the detected infrared radiance, data from a look-up table and measured parameters including information on the position and/or attitude of the aircraft. The image processing computer is further adapted to display the identified adverse atmospheric conditions as a spatial image on a display.

This is a continuation-in-part of International Application No.PCT/EP2001/056805 with an international filing date of Apr. 28, 2011,and claims priority thereto and the benefit thereof, and also claimspriority to and the benefit of U.S. Provisional Patent Application No.61/350,821, filed Apr. 29, 2010 and Norwegian Patent Application No.20100625 filed Apr. 29, 2010 all of which are fully incorporated byreference herein by their entirety.

FIELD

The disclosure herein relates to an apparatus and method for detectingadverse atmospheric conditions ahead of an aircraft. Such conditions mayinclude volcanic clouds and so the apparatus may, for example, detectsulphur dioxide and particles such as volcanic ash, wind-blown dust andice particles. The disclosure herein also relates to a method ofproducing such an apparatus or essential components thereof.

BACKGROUND

Modern civil aircraft typically cruise at between 30,000 and 40,000feet, which places them above most weather systems, except for toweringcumulonimbus clouds and their associated electrical storms. As these canpresent a hazard, most airliners are equipped with weather radar thatcan detect them and enable the pilot to take avoiding action. However,there remain a number of adverse atmospheric conditions that aredesirable to detect for which suitable avionic systems are not presentlyavailable. These include volcanic ash, toxic gases such as sulphurdioxide gas, wind-blown dust and ice particles.

Volcanic clouds contain silicate ash and gases that are hazardous toaviation. Several encounters between jet aircraft and volcanic ash haveresulted in significant damage due to ingestion of ash into the hotparts of the engine, subsequent melting and fusing onto the turbineblades. Ash can also block the pitot static tubes and affect sensitiveaircraft instruments, as well as abrade the leading edges of parts ofthe airframe structure.

Volcanic gases, principally sulphur dioxide (SO₂), whilst less dangerousto aircraft than volcanic ash, do pose a hazard in themselves. Inaddition, their presence can be used as an indicator of volcanic ash asthese substances are often co-located and are transported together byatmospheric winds. SO₂ clouds from volcanoes will react with watervapour in the atmosphere to produce sulphuric acid which can damageaircraft.

Another important gas in volcanic clouds is water vapour (H₂O gas).Water vapour occurs in copious amounts in volcanic clouds, eitherthrough entrainment of ambient air or from water from the volcanicsource (e.g. sea water is a common source for volcanoes on islands or incoastal regions). Once in the atmosphere, the water vapour can condenseon ash which act as nuclei, rapidly forming ice with a much smallerradius than ice in normal meteorological clouds. These abundant,small-sized ice particles are hazardous to aircraft because the rapidmelting of the ice when in contact with the hot engines releases the ashnuclei which then fuse onto the turbine blades, affecting the engineperformance and potentially causing the engine to stop.

Damage to aircraft resulting from encounters with volcanic clouds can becounted in the millions of dollars. Most serious aircraft encounterswith ash clouds have been at cruise altitudes, but there is also ahazard to aircraft at airports affected by volcanic ash. These airportsare usually close to an active volcano but they can also be at somedistance from the source of the eruption due to atmospheric transportthat brings ash into the region.

The cost of ash hazards to airport operations is not known, but must besignificant if the costs include those due to delays to landings andtake-offs as well as re-routing costs incurred by airline operators. TheApril 2010 eruption of Eyjafjallajoekull in Iceland is estimated to havecost the airline industry approximately US$2 billion.

Although there are currently no regulatory requirements for airportoperators to provide warnings of ash hazards, warnings are issued basedon information from volcano observatories, meteorological advisoriesand, in some cases, radar observations of eruption columns. Radarinformation is generally only reliable at the start of an eruption whenthe ash cloud is thick and usually such information is only available atairports in close proximity to an erupting volcano. For airports distantfrom the source of ash there are few direct observations available. Someobservations come from satellite systems and other sources ofinformation come from trajectory forecasts based on wind data and cloudheight information. Much of this information is sporadic and untimelyand there is a need for better detection systems.

Whilst volcanic clouds are the best known example (apart from thunderclouds) of an adverse atmospheric condition, there are other suchhazards. For example, other non-ash particles can also under the rightconditions initiate ice particle formation when they form nuclei aroundwhich water freezes. Ice crystals may accrete within turbine engines andare believed to be the cause of a number of power loss events. Inaddition, there is the possibility of toxic or otherwise dangerousgasses being emitted from industrial sources.

Jet aircraft at cruise altitudes (i.e. above 15,000 feet), travelrapidly (>500 km hr⁻¹) and currently do not have a means for detectingvolcanic cloud hazards ahead. Because of the high speed, a suitabledetection method must be able to gather information rapidly and providean automated alert and be capable of distinguishing volcanic substancesfrom other substances in the atmosphere (e.g. meteorological clouds ofwater and ice).

U.S. Pat. No. 5,654,700 proposes a volcanic cloud detection system whichdisplays the position of a volcanic cloud relative to the aircraft'sposition and thereby enables the aircraft to route around the cloud. Thesystem operates by comparing the absolute and relative values ofbrightness temperatures detected at a number of specific infra-redwavelengths to certain threshold values. However, there is no disclosureabout how these threshold values are determined, except that they arecalculated using a microprocessor and depend on the altitude andattitude of the aircraft.

SUMMARY

In one exemplary embodiment, there is provided a method of detecting anadverse condition in the atmosphere ahead of an aircraft in flight,comprising:—

-   -   a) creating and/or utilizing a model of the atmosphere based on        simulation of infrared radiative transfer characteristics of a        clear atmosphere combined with infrared radiative transfer        characteristics of the adverse condition;    -   b) using an infrared sensor mounted on the aircraft, determining        a value corresponding to the brightness temperature at a given        frequency as viewed ahead of the aircraft;    -   c) based on the aircraft's altitude and attitude, using the        model to determine whether the brightness temperature value        determined in step (b) is indicative of the atmosphere ahead of        the aircraft having the adverse condition; and    -   d) where the adverse condition is indicated in step (c),        providing an alert to the presence of the adverse condition.

It will be appreciated that, as in U.S. Pat. No. 5,654,700, thebrightness temperature value

determined in step (b) will typically be at a frequency that ischaracteristic of the adverse condition. Likewise, brightnesstemperature values may be determined at a plurality of frequenciescorresponding to a single adverse condition or to a plurality of adverseconditions. For example, they may relate to different substances presentin the atmosphere. These may relate, as discussed above, to a commonunderlying cause, such as a volcanic eruption.

Thus, in one exemplary embodiment, the system processes brightnesstemperature values using data obtained from a radiative transfer modelof the atmosphere. Such a model can be created that provides accuratedata corresponding to views from an aircraft at any altitude and takinginto account its attitude (e.g. pitched up or down). This is importantbecause it is known that the background temperature varies significantlydepending on whether the aircraft is directed towards space or the earthfor example. Also, such a model can be created to cover a range ofinfrared frequencies and so the method is readily applicable to thedetection and analysis of brightness temperatures at a number offrequencies. Likewise, it is applicable to the detection of a number ofadverse conditions, or at a plurality of frequencies which arecollectively indicative of a given adverse condition. For example, as isknown from U.S. Pat. No. 5,654,700, a pair of brightness temperaturesrelating to different frequencies may be compared and an indication ofan adverse condition provided if they differ by more than a givenamount.

The radiative transfer model may be highly detailed and therefore largeand therefore it will often be impracticable to store the model itselfin any practical apparatus. Furthermore, the model itself will containfar more data than is required to perform step (c). Therefore, the dataderived from the model can be stored in memory and said data is used inthe determination of step (b).

In particular, the data can be stored as a look-up table, which may, forexample, be indexed by aircraft altitude and attitude. Where a pluralityof frequencies are used, it will also be indexed by frequency.

Although it is possible to store brightness temperatures correspondingto adverse conditions and to compare them to measured brightnesstemperatures, in many cases it will be more convenient to storethreshold values. The use of thresholds is also useful because, as inU.S. Pat. No. 5,654,700, some adverse conditions are identified basedupon functions of one or more brightness temperatures. Thus, the datacomprises threshold values indicative of the presence of the adversecondition and in particular an alert is provided if a function of thebrightness temperature exceeds the threshold value obtained from thelook-up table for the altitude and attitude of the aircraft and for thefrequency(ies) at which the brightness temperature(s) were determined.The threshold values can also be determined to allow for statisticalvariation and error so that a balance is struck between sensitivity andfalse alarms.

As an example, the function may comprise a difference between thebrightness temperature at a first frequency and the brightnesstemperature at a second frequency. The magnitude of the threshold valuemay then indicate a minimum difference between the measured brightnesstemperatures that is regarded as indicating, to an appropriate level ofconfidence, the presence of an adverse condition.

Note that the term “brightness temperature value” is used to indicate aquantity that corresponds, at least substantially, in a known manner, tothe actual brightness temperature. Needless to say, units of measurementare arbitrary. Furthermore, brightness temperature values need not havea linear relation to brightness temperature.

Another exemplary embodiment concerns a method of manufacturing anapparatus for the detection of an adverse condition ahead of an aircraftin flight, the method comprising:

-   (i) providing an electronic data processor including a    microprocessor and memory;-   (ii) creating a model of the atmosphere based on simulation of    infrared radiative transfer characteristics of a clear atmosphere    combined with infrared radiative transfer characteristics of the    adverse condition;-   (iii) storing data derived from the model in the memory;-   (iv) providing input means for communicating to the data processor    data corresponding to the brightness temperature detected by an    infrared sensor, aircraft altitude and aircraft attitude.-   (v) providing software capable of using the data from steps (iii)    and (iv) to determine whether an adverse atmospheric condition    exists ahead of an aircraft.

In particular, the data derived from the model may be stored in alook-up table, as discussed above. The apparatus can be arranged tooperate according to the preferred features set out above. Anotherexemplary embodiment concerns an apparatus provided by this method ofmanufacture, and in particular to one having the preferred methods ofoperation.

The infrared sensors can provide spatially resolved data so that anoutput can be provided in the form of an image. Thus, the method as setout above may be applied to each pixel in the image. Consequently, oneexemplary embodiment may provide not just an indication that an adversecondition exists ahead of the aircraft, but a display indicating itsrelative position to the aircraft's flight path.

The sensors can be collocated uncooled microbolometer cameras. Inaddition, the data processor can be arranged to determine brightnesstemperatures from the detected infrared radiance.

Such an apparatus is advantageous in that it detects adverse atmosphericconditions, in particular caused by volcanoes, and visualizes them forthe crew of the aircraft. The embodiments disclosed herein areparticularly useful for detecting volcanic clouds. For example, oneexemplary embodiment can be arranged to enable the rapid detection ofvolcanic substances ahead of a jet aircraft at cruise altitudes and thesimultaneous detection and discrimination of volcanic ash, SO₂ gas andice-coated ash particles. This exemplary embodiment provides algorithmsand processes for converting raw camera data to identify ash, SO₂ gasand ice coated ash.

The system may also include one or more external blackened shuttersagainst which the infrared sensors are pre-calibrated for providingin-flight calibration values.

The system provides a statistical alert based on analysis of imagesdetermined to show an adverse condition of ash, sulphur dioxide orice-coated ash. The statistical alert uses spatial and temporalinformation and can be tuned according to in-flight tests to reducefalse-alarms and ensure robustness.

As well as the system being arranged to detect at least the threevolcanic substances (ash, SO₂ and ash coated ice particles) in the airahead of the aircraft by a remote method, and in addition it is capableof discriminating these from other meteorological clouds of waterdroplets and ice.

Another exemplary embodiment also more generally provides a system fordetecting adverse atmospheric conditions ahead of an aircraft, includinga plurality of infrared cameras mounted on the aircraft, wherein: theinfrared cameras are adjusted to spatially detect infrared radiance indifferent bands of infrared light; each camera is connected to an imageprocessing computer that processes and combines the images, wherein eachof the cameras is provided with a respective filter adjusted to filterinfrared light with a bandwidth corresponding to infrared bandwidthcharacteristics of an adverse atmospheric condition from a set ofadverse atmospheric conditions; and the image processing computer isadapted to identify and display adverse atmospheric conditions, saididentifying being based on threshold conditions and using the detectedinfrared radiance and measured parameters including information on theposition and/or attitude of the aircraft.

Another exemplary embodiment provides a system for detecting adverseatmospheric conditions ahead of an aircraft, including a plurality ofinfrared cameras mounted on the aircraft, wherein: the infrared camerasare adjusted to spatially detect infrared radiance in different bands ofinfrared light, each camera is connected to an image processing computerthat processes and combines the images, and generates video displaysignals for producing a video display which indicates the position ofthe adverse atmospheric conditions relative to the aircraft; each of thecameras is provided with a respective filter adjusted to filter infraredlight with a bandwidth corresponding to infrared bandwidthcharacteristics of an adverse atmospheric condition from a set ofadverse atmospheric conditions; the image processing computer is adaptedto identify adverse atmospheric conditions, said identifying being basedon threshold conditions and using the detected infrared radiance, datafrom a look-up table and measured parameters including information onthe position and/or attitude of the aircraft; and the image processingcomputer is further adapted to display the identified adverseatmospheric conditions as a spatial image on a display.

Another embodiment provides a method for detecting adverse atmosphericconditions ahead of an aircraft and displaying said adverse atmosphericconditions, comprising spatially detecting infrared radiance indifferent bands of infrared light using a plurality of infrared cameras;and, for each camera: i) Filtering the infrared radiation with a filteradjusted to filter infrared light with a bandwidth corresponding toinfrared bandwidth characteristics of an adverse atmospheric conditionin a set of adverse atmospheric conditions; ii) identifying likelyoccurrences of adverse atmospheric conditions based on thresholdconditions and using the detected infrared radiance, data from a look-uptable and measured parameters including information on the positionand/or attitude of the aircraft; and iii) processing the identifiedlikely occurrences of adverse atmospheric conditions to create a spatialimage.

In one embodiment, the method further comprises the step of iv)combining the image with images from other cameras and information onthe aircraft flight path.

The adverse atmospheric conditions can include volcanic ash, ice coatedash, water vapour and sulphur dioxide. The measured parameters caninclude pitch angle and ambient temperature.

In a further aspect, this embodiment provides a system for detectingvolcanic clouds ahead of an aircraft, including one or more infraredcameras mounted on the aircraft, the infrared cameras are adjusted tospatially detect infrared radiance in different bands of infrared light,each camera is connected to an image processing computer that processand combines the images, combining them with flight path informationfrom the aircraft and generates video display signals for producing avideo display which indicates the position of the adverse conditionsrelative to the aircraft; characterized in that each of the cameras isprovided with a respective filter adjusted to filter infrared light witha bandwidth corresponding to infrared bandwidth characteristics of oneof the volcanic species in a set of volcanic species, and that the imageprocessing computer is adapted to identify and display species as aspatial image on a display by means of threshold look-up tables for therespective species mapping thresholds for the infrared radiance, abovewhich species are likely to occur, with measured parameters.

In another exemplary embodiment, a method for detecting a volcanic cloudahead of an aircraft and displaying said cloud, processing informationfrom one or more infrared cameras spatially detecting infrared radiancein different bands of infrared light, combining the information withflight path information from the aircraft characterized in the steps offor each camera: i) Filtering the infrared radiation with a filteradjusted to filter infrared light with a bandwidth corresponding toinfrared bandwidth characteristics of one of the volcanic species in aset of volcanic species; ii) identifying likely occurrences of speciesby looking up detected infrared radiance values in a threshold look-uptable mapping thresholds for the infrared radiance, above which speciesare likely to occur, with measured parameters; iii) processing theidentified likely occurrences of species to create a spatial image.

BRIEF DESCRIPTION OF THE DRAWINGS

Exemplary embodiments will now be described, by way of example only,with reference to the accompanying drawings, in which:—

FIG. 1 is a schematic diagram of a volcanic cloud detector according toan exemplary embodiment;

FIG. 2 is a schematic diagram illustrating the detector array of FIG. 1mounted;

FIG. 3 is a schematic diagram of an infrared camera that forms part ofthe detector array of the embodiment;

FIGS. 4( a) to (f) shows an ash cloud on the display of the embodiment;

FIG. 5 shows a series of plots of infrared radiance against wave numberfor the horizontal path ahead of a cruising aircraft at three differentaltitudes in a clear atmosphere; and

FIG. 6 shows a diagram of line strengths for the two bands of SO₂ at 8.6μm and 7.3 μm. The response functions for the filters of the system arealso shown.

DETAILED DESCRIPTION OF CERTAIN EXEMPLARY EMBODIMENTS

As shown in FIG. 1, the volcanic cloud detection apparatus of theembodiment comprises a detector array 1 and its associated electronics2, a data processing unit 3, memory 4 and a display unit 5 that providesoutput data to an aircraft pilot. Computer instructions for implementingany of the methods described herein are capable of being stored on thememory 4. These instructions can be executed by the processing unit 3.

As may be seen from FIG. 2, detector array 1 comprises five infrareddetectors 6, which are infrared sensitive cameras and are locatedadjacent and parallel to each other within a housing 7. At one end ofthe housing, a mechanically driven protective shutter 8 is provided,which is closed when the apparatus is not in use. This is blackened andis provided with a heater (not shown) for use in calibration of thecamera, as will be explained below. The shutter also serves the purposeof providing protection against debris and dirt directed toward thecamera during take-off and landing, when the system is deactivated. Agermanium glass window 9 is located behind the shutter to provideprotection from debris while in viewing mode. The cameras 6 are locatedbehind the germanium window, with their signal 10 and power 11 linesextending from the back of the housing 7. The housing 7 also containsthe camera electronics and software unit 2 including a frame grabber andrelated computer hardware.

The cameras are wide-field-of-view, rapid sampling, imaging, uncooledmicrobolometer cameras as shown in FIG. 3. Each of these detects adifferent narrow band (0.5-1.0 μm) of infrared radiation within theregion of 6-13 μm. These enable the detection of ash, water vapour, SO₂gas and ice coated ash.

Each camera 6 comprises a housing 12 which contains detector 13, lens14, and narrow band filter 15. The housing is provided with an infraredtransmissive germanium glass window 16 and the window is covered by acamera shutter 17. Infrared radiation from ahead of the aircraft entersthe filter 15, is focused through the camera lens 14 and falls on thedetector array 13.

In a microbolometer, infrared radiation strikes the detector 13, heatingit, and thus changing its electrical resistance. This resistance changeis measured and processed into temperatures (as described below) whichcan be used to create an image. Unlike other types of infrared detectingequipment, microbolometers do not require cooling.

The detector 13 has 640×512 pixels and has a noise equivalenttemperature difference of 50 mK (or better) at 300 K in the 10-12 μmregion, and provides sampling rates up to 60 Hz. It is sensitive toinfrared radiation within the region 6-13 μm.

Narrowband filters 15 are placed over the lens 14 of each camera 6 torestrict spectral content of the radiation that reaches detector 13 to anarrow band. Each camera in the array is sensitive to a different band.The cameras share the same field of view ahead of the aircraft andtherefore, in principle, multiple, simultaneous narrowband infraredimages can be acquired by the array 1 in real-time.

To protect the filter and lens while the system is viewing ahead of theaircraft, an IR transparent window 16 (e.g. Germanium glass) is attachedbetween the shutter and filter. The shutter is temperature controlledand blackened on the side facing the optics.

A detector electronics module 2 is provided within the housing 7 of thedetector array. This contains the components necessary to pre-processthe data output from each detector 13.

The cameras 6 are pre-calibrated prior to installation on the aircraftso that each camera registers the same digital signal when exposed tothe same amount of infrared radiation. This can be achieved by pointingeach camera, without its filter, at a known source of infrared radiationhaving a known constant temperature and recording the digital signalfrom each pixel of each camera. A calibration look-up table is thencreated in non-volatile memory within module 2 by varying the sourcetemperature through the range 210 to 300 K, in steps of 10 K (forexample) for each camera, giving a table of 640×512×10×2 values,assuming a linear calibration. Thus, the calibration look-up tableenables a radiance value to be provided for a given digital output fromeach pixel. (Radiance is assumed to have a linear relationship to thesignal counts). This process is repeated for each narrowband filterused. Once on board the aircraft, intermittent re-calibrations areperformed using the blackened shutter 8. Optionally, a second shuttercould be used to provide a second calibration point in a linearcalibration equation. This is heated and placed in front of the filter.The digital counts corresponding to the known (controlled) temperatureof the shutter are then recorded.

The calibrated radiance data for each pixel of all five cameras istransmitted from detector electronics module 2 to processing unit 3.This comprises a conventional computer architecture having amicroprocessor and memory 4, which includes a look-up table. Themicroprocessor runs software that implements a number of algorithmswhich, using data from the look-up table, determine whether the outputsfrom the cameras 6 are indicative of one or more volcanic cloud species,or another hazardous condition. When a hazardous condition is detected,the processing unit activates display 5. The operation of the algorithmsand the resulting output display will now be described in more detail.

The system is de-activated until the aircraft reaches cruise altitude.In deactivated mode the shutter is closed. Before activation apre-calibration cycle for the system (all five cameras) is conducted.The shutter is then opened and the system begins to collect images.

Commercial cameras can sample as fast as 60 Hz and this is the preferredsampling rate (or higher). However, some export restrictions apply tosome cameras and this means lower sampling rates may apply. In thedescribed embodiment, a sampling rate of 8 Hz is used, as at thisfrequency there are no export restrictions. (The basic principle isunchanged when using a higher sampling frequency).

Each camera provides eight images of size N columns by M lines (whereN=640 and M=512) every second. The processor unit first converts thecalibrated radiance values obtained from the camera electronics to abrightness temperature (BT_(i,j,k)), where k represents the cameranumber and k=1, 2, 3, 4 or 5, in the current system, and i and j arecolumn and line numbers, respectively (i≦N, j≦M). The brightnesstemperature is determined from:

$R_{i,j,k} = \frac{c_{1}v_{k}^{3}}{^{c_{2}{v_{k}/{BT}_{i,j,k}}} - 1}$

Where:

-   -   R_(i,j,k) is the radiance at pixel i, line j and filter k    -   v_(k) is the central wave number for camera filter k    -   BT_(i,j,k) is the brightness temperature    -   c₁ and c₂ are the Einstein radiation constants

Camera images may be averaged in order to reduce noise and improve thesignal-to-noise ratio of the system.

For illustration purposes only, we shall concentrate on one image pixeland assume that all other pixels can be treated in the same manner,noting that the calibration look-up table is different for every pixel.The data for one pixel consists of the measurements: BT1, BT2, BT3, BT4and BT5, where these represent brightness temperatures from each of thefive cameras (e.g. BT1 is the brightness temperature for that pixel incamera 1 which has filter 1).

The system is linked into the aircraft instrument data stream so thatGPS coordinates, altitude (z), longitude (l), latitude (q), heading (h),direction (d), roll (r), yaw (y), pitch (x), time (t), speed over theground (v), wind speed (w) and local ambient temperature (Ta) areavailable at a sampling rate of at least 1 s⁻¹ and preferably faster.

The five values of brightness temperature are compared to each otherand/or to the ambient temperature using a series of algorithms. Eachalgorithm uses one or more predetermined threshold values stored in alook-up table in memory 4. These values are determined from a modelatmosphere that is based on radiative transfer calculations, as will bedescribed further below. Note that all temperatures are in kelvins.

In the embodiment, the system uses filters at the following centralwavenumbers (in cm⁻¹):

TABLE 1 Filter specifications for an exemplary embodiment. Central waveBandwidth NEDT Filter number (cm⁻¹) (cm⁻¹) (mK) Purpose 1 1410 100 200H2O 2 1363 100 200 SO2 3 1155 100 200 SO2/ash 4 929 60 100 Ash/ice 5 83060 100 Ash/ice

Ash Detection Algorithm

A pixel is declared to be ash if both of the following conditions aremet at each instance:

DT1_(Ash)=(BT4−BT5)/BT4>T1_(Ash)(Ta,r,y,x)/Ta  (1)

DT2_(Ash)=(BT3−BT5)/BT3>T2_(Ash)(Ta,r,y,x)/Ta  (2)

Where T1 _(Ash) and T2 _(Ash) are threshold values of temperaturedifference that are both a function of a set of parameters, includingambient temperature (Ta) and realistic aircraft roll, pitch and yawvalues. Note that DT1 _(Ash) and DT2 _(Ash) are non-dimensionalquantities and are strictly indices. Ta may vary across the image—i.e.it need not be the same for all pixels. An alert is sounded if asequence of 8 consecutive occurrences of conditions (1) and (2) happenfor a pre-defined fraction of the total image. A value of 5% of thetotal number of pixels in the difference image is used, but this can betuned as necessary—a lower value set if the aircraft is operating inairspace declared, or likely to be influenced by volcanic ash; a highervalue in unaffected areas.

H₂O Detection Algorithm

A pixel is declared to be water vapour affected if the followingconditions are met at each instance:

DT _(wv) =BT1−Ta>T _(wv)(Ta,r,y,x)  (3)

Where T_(wv) is a threshold value of temperature difference that is afunction of a set of parameters, including ambient temperature (Ta), andrealistic aircraft roll, pitch and yaw values.

-   -   No alert is sounded, but T_(wv) is used with the ice algorithm        if that alert is sounded.

Ice-Coated Ash (ICA) Detection Algorithm

A pixel is declared to be ICA if the following conditions are met ateach instance,

DT _(ICA)=(BT4−BT5)/BT 4<T _(ICA)(Ta,r,y,x)/Ta  (4)

Where T_(ICA) is a threshold value of temperature difference that is afunction of a set of parameters, including ambient temperature (Ta), andrealistic aircraft roll, pitch and yaw values.

An alert is sounded if a sequence of 8 consecutive occurrences ofcondition (4) happen for a pre-defined fraction of the total image. Avalue of 5% of the total number of pixels in the difference image isused, but this can be tuned as necessary—a lower value set if theaircraft is operating in airspace declared or likely to be influenced byvolcanic ash; a higher value in unaffected areas. When the alert issounded condition (3) is checked and if this condition is met, the pixelis confirmed to be ICA. The use of the water vapour condition isentirely novel and reduces the false alarm rate for detecting hazardoussmall-sized ice-coated ash particles.

SO₂ Detection Algorithm

A pixel is declared to be SO₂ if the following conditions are met ateach instance,

DT1_(SO2)=(BT1−BT2)/BT1<T1_(SO2)(Ta,r,y,x)/Ta  (5)

DT2_(SO2)=(BT3−BT5)/BT3<T2_(SO2)(Ta,r,y,x)/Ta  (6)

Where T1 _(SO2) and T2 _(SO2) are threshold values of temperature thatare a function of a set of parameters including ambient temperature(Ta), and realistic aircraft roll, pitch and yaw values.

An alert is sounded if a sequence of 8 consecutive occurrences ofconditions (5) and (6) happen for a pre-defined fraction of the totalimage. A value of 5% of the total number of pixels in the differenceimage is used, but this can be tuned as necessary a lower value set ifthe aircraft is operating in airspace declared to be or likely to beinfluenced by volcanic ash; a higher value is used in unaffected areas.

An example of the display shown to the crew for the detection of an ashcloud is shown in FIG. 4. (The actual display uses multiple colours).This is based on an ash cloud composed of silicate material and showsthe DT1 _(Ash) signal for 6 frames separated by a constant short timedifference from two cameras imaging ahead of the aircraft. Highestconcentrations of ash are indicated in red (dark in FIG. 4); thebackground sky is shown in light purple (or light grey in FIG. 3). Asthe aircraft approaches the hazard, the pilot can alter the heading ofthe aircraft to avoid it.

As discussed above, the threshold values used in the algorithm arepre-computed values stored in a look-up table in memory 4. They areobtained from a detailed radiative transfer model of the atmosphereutilizing geometrical considerations appropriate for viewing in theinfrared region (6-13 μm) from an aircraft. The model uses a threedimensional ‘monte carlo’ approach that allows arbitrary viewinggeometry and for the medium (i.e. the substances present in theatmosphere) to be specified flexibly. Thus, the presence of ash,ice-covered ash, mixtures, water clouds, water vapour, etc may all bespecified.

The model also calculates an “environmental” temperature of each of theimage pixels based on the ambient temperature sensed by the aircraftinstruments. Versions of the model are created with and without avolcanic cloud. A comparison of the clear atmosphere model with one inwhich a volcanic cloud is simulated identifies the “signature”characteristics which occur only when a volcanic cloud is present.Suitable threshold values can then be determined.

The look-up table is a multi-dimensional array which is indexed usingambient temperature and aircraft altitude and attitude data so that agiven, predetermined threshold value is provided for sets of the otherparameters.

FIG. 5 shows a horizontal path simulation of the radiance of the clearatmosphere from 700-1600 cm⁻¹ at three different flight altitudes (thelowermost radiance curve is an altitude of 9.5 km, the middle is 4.5 km,and the uppermost is 1.5 km). At 9.5 km the atmosphere appears verycold—the equivalent blackbody temperature of the horizontal path isabout 227 K. Notice that the radiance curves change with altitude andhence with ambient temperature as determined by the on board aircraftinstrumentation and used by the detection algorithm. (It could equallyuse altitude instead of the temperature, but the temperature is a morerobust measure). Any volcanic cloud placed between the aircraft and thecold background will alter the radiance received by the system in aknown way. Thus the spectral content of the radiation then containssignatures of ash, SO₂, H₂O and ice-coated ash particles. The thresholdvalues obtained from the radiative transfer model are stored in thelook-up table.

The ash signal in these spectra is characterised by a higher brightnesstemperature in filter 4 (BT4) than in filter 5 (BT5), when viewing acold background. The threshold values are determined by using refractiveindex data for silicates and scattering calculations are based onmeasured particle size distribution for particles with radii in therange 1-20 μm, according to the art. Generally, the camera array 6 willlook in the horizontal or slightly upwards (aircraft usually have a 3°pitch angle upwards). However, the aircraft may pitch downwards, inwhich case the background temperature might change from a coldbackground to a warm background. It is for this reason that the aircraftaltitude must be considered when determining threshold values. In thecold background case, the ash signature is identified by BT4<BT5. Inother words, the threshold will have the opposite sign to the pitch-upequivalent. The look-up table is therefore constructed in such a waythat the pitch angle and ambient temperature are accounted for.Additionally, the roll and yaw angles are compensated for, althoughthese have only a minor influence on the detection algorithm. Extrafail-safe thresholds are also incorporated into the detection algorithmby utilizing a filter near 8.6 μm that has sensitivity to volcanic ash.

The operation of the ice-coated ash algorithm is similar to the ashalgorithm, except the threshold look-up table is now determined usingdata for ice (refractive indices and scattering data for smallparticles, radii<30 μm). In the case of small ice particles, BT4<BT5 forviewing into a cold background (the opposite to ash without an icecoating). Background conditions are accounted for in a similar way tothat used for the ash detection.

Normalisation of the temperature differences is done to provide somerobustness and to make the detection independent of the ambient airtemperature.

SO₂ and H₂O threshold look-up tables are also used. SO₂ has very strongabsorptions near to 8.6 μm and 7.3 μm as FIG. 6 illustrates. Theprinciple of detecting SO₂ has been described earlier and is based onradiative transfer calculations assuming the line strengths andtransmissions applicable to the case of an atmosphere loaded with SO₂.Under normal conditions SO₂ has an extremely low abundance (<10⁻³ ppm),and so detection of SO₂ using these absorptions features is veryeffective in the case of volcanic clouds ahead of an aircraft.

1. A method of detecting an adverse condition in the atmosphere ahead ofan aircraft in flight, comprising: a) utilizing a computer model of theatmosphere based on a simulation of infrared radiative transfercharacteristics of a clear atmosphere combined with infrared radiativetransfer characteristics of the adverse condition; b) using an infraredsensor mounted on the aircraft to determine a value corresponding to thebrightness temperature at a given frequency as viewed ahead of theaircraft; c) based on the aircraft's altitude and attitude, using themodel to determine by a processor whether the brightness temperaturevalue determined in step (b) is indicative of the atmosphere ahead ofthe aircraft having the adverse condition; and d) where the adversecondition is indicated in step (c), providing by the processor an alertto the presence of the adverse condition.
 2. A method as claimed inclaim 1, wherein data derived from the model is stored in memory andsaid data is used in the determination of step (b).
 3. A method asclaimed in claim 2, wherein the data is stored as a look-up table.
 4. Amethod as claimed in claim 3, wherein the look-up table is indexed byaircraft altitude and attitude.
 5. A method as claimed in claim 4,wherein the data comprises threshold values indicative of the presenceof the adverse condition.
 6. A method as claimed in claim 5, wherein analert is provided if a function of the brightness temperature exceedsthe threshold value obtained from the look-up table for the altitude andattitude of the aircraft and for the frequency at which the brightnesstemperature was determined.
 7. A method as claimed in claim 6, whereinthe function comprises a difference between the brightness temperatureat a first frequency and a further brightness temperature at a secondfrequency.
 8. A method of manufacturing an apparatus for the detectionof an adverse condition ahead of an aircraft in flight, the methodcomprising: (i)—providing an electronic data processor including amicroprocessor and memory; (ii)—utilizing a computer model of theatmosphere based on a simulation of infrared radiative transfercharacteristics of a clear atmosphere combined with infrared radiativetransfer characteristics of the adverse condition; (iii)—storing dataderived from the model in the memory; (iv)—linking the data processordata to the brightness temperature detected by an infrared sensor,aircraft altitude and aircraft attitude; and (v)—providing softwarecapable of using the data from steps (iii) and (iv) to determine whetheran adverse atsmopheric condition exists ahead of an aircraft.
 9. Amethod as claimed in claim 8, wherein the data derived from the model isstored in a look-up table.
 10. A method as claimed in claim 9, whereinthe data comprises threshold values indicative of the presence of theadverse condition.
 11. A method as claimed in claim 9, wherein an alertis provided if a function of the brightness temperature exceeds thethreshold value obtained from the look-up table for the altitude andattitude of the aircraft and for the frequency at which the brightnesstemperature was determined.
 12. A method as claimed in claim 11, whereinthe function comprises a difference between the brightness temperatureat a first frequency and a further brightness temperature at a secondfrequency.
 13. A method as claimed in claim 8, wherein a set of adverseatmospheric conditions is detected, the set including volcanic ash, icecoated ash, water vapour and/or sulphur dioxide.
 14. A method as claimedin claim 13, wherein the identification of water vapour is used toconfirm an identification of ice coated ash.
 15. A method as claimed inclaim 8, wherein brightness temperature is determined from detectedinfrared radiance.
 17. A method as claimed in claim 8, comprising theuse of one or more external blackened shutters against which theinfrared sensors may be calibrated.
 18. An apparatus comprising adetector array including at least one infrared sensor, a processingunit, and a display unit wherein the apparatus is configured to performa method of detecting an adverse condition in the atmosphere ahead of anaircraft in flight, the method comprising: a) utilizing a computer modelof the atmosphere based on a simulation of infrared radiative transfercharacteristics of a clear atmosphere combined with infrared radiativetransfer characteristics of the adverse condition; b) using the infraredsensor mounted on the aircraft to determine a value corresponding to thebrightness temperature at a given frequency as viewed ahead of theaircraft; c) based on the aircraft's altitude and attitude, using themodel to determine by the processor whether the brightness temperaturevalue determined in step (b) is indicative of the atmosphere ahead ofthe aircraft having the adverse condition; and d) where the adversecondition is indicated in step (c), providing by the processor an alertto the presence of the adverse condition on the display unit.
 19. Theapparatus as claimed in claim 18, wherein data derived from the model isstored in the memory and said data is used in the determination of step(b).
 20. The apparatus as claimed in claim 19, wherein the data isstored as a look-up table.
 21. The apparatus as claimed in claim 20,wherein the look-up table is indexed by aircraft altitude and attitude.22. The apparatus as claimed in claim 21, wherein the data comprisesthreshold values indicative of the presence of the adverse condition.23. The apparatus as claimed in claim 22, wherein an alert is providedif a function of the brightness temperature exceeds the threshold valueobtained from the look-up table for the altitude and attitude of theaircraft and for the frequency at which the brightness temperature wasdetermined.
 24. The apparatus as claimed in claim 23, wherein thefunction comprises a difference between the brightness temperature at afirst frequency and a further brightness temperature at a secondfrequency.
 25. A non-transitory computer readable medium storingcomputer-executable instructions, which when executed by a processorcause the apparatus to perform a method of detecting an adversecondition in the atmosphere ahead of an aircraft in flight, comprising:a) utilizing a computer model of the atmosphere based on a simulation ofinfrared radiative transfer characteristics of a clear atmospherecombined with infrared radiative transfer characteristics of the adversecondition; b) using an infrared sensor mounted on the aircraft todetermine a value corresponding to the brightness temperature at a givenfrequency as viewed ahead of the aircraft; c) based on the aircraft'saltitude and attitude, using the model to determine by the processorwhether the brightness temperature value determined in step (b) isindicative of the atmosphere ahead of the aircraft having the adversecondition; and d) where the adverse condition is indicated in step (c),providing by the processor an alert to the presence of the adversecondition.
 26. The non-transitory medium as claimed in claim 25, whereindata derived from the model is stored in the memory and said data isused in the determination of step (b).
 27. The non-transitory medium asclaimed in claim 26, wherein the data is stored as a look-up table. 28.The non-transitory medium as claimed in claim 27, wherein the look-uptable is indexed by aircraft altitude and attitude.
 29. Thenon-transitory medium as claimed in claim 28, wherein the data comprisesthreshold values indicative of the presence of the adverse condition.30. The non-transitory medium as claimed in claim 29, wherein an alertis provided if a function of the brightness temperature exceeds thethreshold value obtained from the look-up table for the altitude andattitude of the aircraft and for the frequency at which the brightnesstemperature was determined.
 31. The non-transitory medium as claimed inclaim 30, wherein the function comprises a difference between thebrightness temperature at a first frequency and a further brightnesstemperature at a second frequency.